(1) Field of the Invention
The present invention relates to cooling the walls of shaft furnaces. More specifically, this invention is directed to apparatus for use in the circulation of a coolant between the external steel shell and the internal refractory lining of a blast furnace. Accordingly, the general objects of the present invention are to provide novel and improved methods and apparatus of such character.
(2) Description of the Prior Art
A shaft furnace, for example a high-capacity blast furnace of the type used in the steel industry, will typically have a wall defined by an outer metal jacket and an inner lining of refractory brickwork. Since the refractory brickwork will deteriorate with time, it is desirable to provide means for at least partly protecting the inner surface of the metal jacket. This protection means will take the form of a cooling system whereby a coolant may be circulated through at least portions of the region between the inner surface of the external metal jacket and the refractory brickwork. Two types of cooling systems are known in the prior art. These known art cooling systems are generally referred to as being of either the "cooling box" type or the "stave cooler" type. In either case, the cooling system is comprised of a plurality of individual cooling elements which function to both prevent overheating of the inner surface of the metal blast furnace external jacket and to cool, and thus extend the life of, the refractory lining.
In the "cooling box" type of system, a large number of spaced and inwardly extending "boxes", which may be formed of copper, are installed in the furnace wall. In the case of a high-capacity blast furnace, it has been known to employ up to 1700 cooling boxes. The "cooling boxes" are each equipped with a single multiple internal circuit for the circulation of a coolant, which is typically water, and the "cooling boxes" are customarily arranged in series and inter-connected outside of the furnace jacket. Tests have shown that the degree of cooling of the refractory brickwork achieved with a set of cooling boxes may be plotted as curves, which resemble cycloids, with the "hot spots" being close to the jacket and between individual boxes and with the amount of cooling achieved varying over the furnace wall in the vertical direction.
The "stave coolers", which are also know as "cooling plates", consist of rectanglar cast iron panels which are traversed internally, and parallel to their main faces, by a number of conduits. The conduits may be oriented parallel to one another and to the axis of the furnace or arranged in coils. The "stave coolers" vary in thickness from approximately 16 cm to 25 cm when new and are bolted to the inner surface of the jacket and therefore form a complete lining for the jacket. The inner surfaces of the stave coolers, i.e., the surfaces facing towards the interior of the furnace, may be provided with a lining of refractory brick. The space between adjacent cooling plates and any gap which may be left between the furnace jacket and the refractory lining will be respectively filled with a mortar and with a refractory paste. The conduits provided in the "stave coolers" are interconnected vertically in series with the connection between the conduits of adjacent vertically spaced plates being made outside of the furnace jacket.
The operational life of a shaft furnace, i.e., the time between major overhauls, is a function of the durability of the inner refractory brickwork and this is a function of the efficiency of the furnace cooling system. The refractory brickwork, as is well known, undergoes considerable wear as a function of mechanical and thermal stresses and also as a result of chemical reactions. The prior art cooling systems have possessed certain deficiencies which have limited their ability to provide cooling to the refractory brickwork, thus enhancing its service life, and to simultaneously provide protection for the furnace jacket.
In the prior art box-type cooling systems the wear and erosion of the refractory brickwork is approximately in accordance with the aforementioned cycloidal or isothermal curves. The "noses" of the individual cooling boxes become increasingly subject to wear as the brickwork suffers wear. The cooling boxes therefore undergo increasing strain which may lead to their destruction. On the other hand, a cooling box system has the attribute of ease of repair since worn boxes may be replaced with comparative ease. Nevertheless, because of the geometrical arrangement of the cooling boxes and the reduction in thickness of the brickwork which occurs with time, the external furnace jacket suffers increasing stresses as a result of temperature differentials and the risk of deformation or premature destruction of the jacket is aggravated. Cooling box type systems involve the further drawback of being unsuitable for use in evaporative type cooling which is finding ever increasing favor since the steam generated during evaporative cooling can be employed for power generation and because evaporative cooling requires less coolant. The inability to use cooling boxes in evaporative cooling results from the fact that the horizontal arrangement of the cooling boxes and variations in the cross section of the circulation channels cause interference with the circulation of the coolant.
"Stave coolers" have the attribute, when compared with cooling boxes, of producing more uniform cooling thereby minimizing the occurrence of "hot spots" resulting from the more localized cooling at separate points as effected in a cooling box system. Also, "stave coolers" may be utilized in a cooling system based upon evaporative cooling. On the other hand, prior art "stave coolers" cannot be dismantled or replaced. In this regard it is to be noted that the refractory brickwork initially provided on the internal surface of the "stave coolers" rapidly disappears in the course of operation of the blast furnace. When this brickwork has disappeared, deposits or "coatings" may form on the internal surface of the plates, and these coatings are periodically eroded and reformed. When the original refractory brickwork has been eroded, the cooling plates have to depend on these coatings for protection against wear. The coatings do not provide a sufficiently durable and effective means of protection partly because they tend to collapse for want of any type of support. In blast furnace operation it has previously been common practice to render inoperative, by means of short circuiting, individual stave coolers which have worn to the point where their internal coolant conduits have been breached. When an excessive number of cooling plates have been removed from the coolant circulation system by such "short-circuiting", it becomes necessary to shut down the furnace for major overhaul.